Method and apparatus for form machining the peripheral edge of spectacle lenses

ABSTRACT

The invention relates to a method and a device for machining the peripheral edge of spectacle lenses and for optionally applying a subsequent facet thereto. The device comprises a driven spectacle-lens holding shaft which holds the spectacle lens and a machining tool which can be moved in a controlled manner in relation to the spectacle-lens holding shaft. The torque of the spectacle-lens holding shaft is measured and said holding shaft is driven at a constant rotational speed if and for the length of time that the torque is less than or equal to a predetermined value. The torque of the spectacle-lens holding shaft is decreased optionally to a standstill if and for the length of time that the torque exceeds the predetermined value.

The invention relates to a method and an apparatus for form machiningthe peripheral edge of spectacle lenses and of subsequently applying, ifneed be, a bevel, having a driven spectacle-lens holding shaft holdingthe spectacle lens and a cutting tool movable in a controlled manner inrelation to the spectacle-lens holding shaft.

In order to carry out the form machining of the peripheral edge ofspectacle lenses as quickly as possible without the risk of breaking ordamaging the spectacle lenses and at the same time definitely avoid asituation in which the spectacle lens clamped in place in thespectacle-lens holding shaft slips through, it is known according to DE196 16 536 A1 of the same applicant for the force with which the cuttingtool is guided against the spectacle lens to be increased in acontrolled manner as a function of the respective radius, from a largeradius to a small radius, in the bearing region of the spectacle lenstouching the cutting tool. In this way, the cutting pressure can be setin such a way that, at a large spectacle-lens radius touching thegrinding wheel, it is just so large that a situation in which thespectacle lens clamped in place in the spectacle-lens holding shaftslips through is just avoided, in which case the cutting pressure can beincreased with decreasing radius, and this increase in turn, on the onehand, depends on the permissible instantaneous torque exerted on thespectacle lens by the cutting pressure, but, on the other hand, must notbecome so large that the spectacle lens is damaged or even destroyed asa result.

In this known method, the cutting pressure can be controlled by means ofa data volume of a computer, which also controls the form machining ofthe spectacle lens by means of this data volume. In particular, thecutting pressure can be controlled by varying the torque transmitted bythe servomotor moving the cutting tool relative to the spectacle-lensholding shaft, in which case the variation in the torque can be carriedout either by means of a torque-controlled servomotor or by means of atorque-controlled coupling between the servomotor and the cutting toolwhich can be fed in.

The method described above and the associated apparatus have provedsuccessful; however, a relatively complicated control is required forthis.

It is known from DE 196 32 340 A1 to measure the torque of the drivemotor of a spectacle-lens holding shaft and/or of a grinding-wheelspindle in a spectacle-lens edging machine and to use it to control thecutting force. A variation in the cutting force is achieved byinfluencing a vertical-adjustment device for the spectacle-lens holdingshaft, which is activated in such a way that a pin descends,specifically in such a way that a predetermined grinding pressure isobtained in the contact region between the raw glass and the grindingwheel.

The method described above has proved successful, but can be simplifiedif the control for the shaping of the spectacle lens is separated fromthe control of the applied pressure.

The problem underlying the invention is likewise to provide a method andan apparatus for form machining the peripheral edge of spectacle lensesand for subsequently applying, if need be, a bevel, with which asituation in which the spectacle lens clamped in place in thespectacle-lens holding shaft slips through is definitely avoided, andwith which the form machining of the peripheral edge of spectacle lensescan be carried out as quickly as possible without the risk of breakingor damaging the spectacle lenses, in which case this method and theapparatus are to be of simple design or construction and are not torequire a complicated control.

Starting from this definition of the problem, it is proposed in a methodof the type mentioned at the beginning that the torque of thespectacle-lens holding shaft be measured, that form machining of theperipheral edge of the rotating spectacle lens be carried out with apredeterminable, adjustable force of the cutting tool if and as long asthe torque is less than or equal to a predetermined, set value, and thatthe cutting-tool force acting on the spectacle lens be reduced byreducing the rotational speed of the spectacle-lens holding shaft, ifneed be until it stops, if and as long as the torque exceeds thepredetermined value.

As a result, the cutting force acting on the spectacle lens clamped inplace in the spectacle-lens holding shaft always remains below apredeterminable value at which the spectacle lens definitely does notslip through. This method has the advantage that it can be integrated ina spectacle-lens edging machine independently of the control of the formmachining, if need be even by retrofitting. For example, the methodaccording to the invention does not depend on whether the form machiningof the spectacle lens is effected in a CNC-controlled manner or by copygrinding with the aid of a form wheel.

The measurement of the torque of the spectacle-lens holding shaft can becarried out in different ways, for example by measuring the torque ofthe drive motor of the spectacle-lens holding shaft. This torque isproportional to the power input, so that the measurement of the torquecan be achieved by measuring the power input of the drive motor.Likewise, the torque of the drive motor can be measured at the housingof the drive motor if this housing is mounted on an elastic support soas to be slightly rotatable and the displacement of the elastic supportis measured.

The rotational speed of the spectacle-lens holding shaft may becontrolled either in such a way that it is reduced in proportion to theamount by which the predetermined value of the torque is exceeded or bythe spectacle-lens holding shaft being stopped for as long as apredetermined value of the torque is exceeded.

According to the invention, the apparatus for form machining theperipheral edge of spectacle lenses and for subsequently applying, ifneed be, a bevel, having a driven spectacle-lens holding shaft holdingthe spectacle lens and a cutting tool movable in a controlled manner inrelation to the spectacle-lens holding shaft, has a measuring device formeasuring the torque of the spectacle-lens holding shaft, and a controldevice for the force, acting on the spectacle lens, of the cutting toolfor reducing this force by reducing the rotational speed of thespectacle-lens holding shaft, if need be until it stops, if and as longas the torque exceeds a predetermined value.

The control device can act on the drive of the spectacle-lens holdingshaft for reducing the rotational speed of the spectacle-lens holdingshaft, if need be until it stops.

The measuring device for measuring the torque preferably has an elasticsupport of the rotatably mounted housing of the drive motor for thespectacle-lens holding shaft, in which case a position encoderinteracting with the control device may be arranged on the support, andthe control device brings about a reduction in the rotational speed ofthe spectacle-lens holding shaft in proportion to the amount by which apredetermined displacement of the position encoder is exceeded.

An especially advantageous embodiment of the apparatus according to theinvention is obtained if the elastic support is prestressed in such away that that a movement does not occur until the prestress is exceeded,i.e. until the predetermined torque is exceeded, and in which anelectrical contact interacts with the elastic support, this electricalcontact interrupting the power supply to the drive motor when theprestress is exceeded.

The method according to the invention and the apparatus can be combinedwith the method described in DE 196 16 536 A1 in such a way that, on theone hand, the cutting pressure is increased in a controlled manner as afunction of the respective radius, from a large radius to a smallradius, in the bearing region of the spectacle lens touching the cuttingtool and, on the other hand, the rotational speed of the spectacle-lensholding shaft is reduced, if need be until it stops, if and as long asthe torque exceeds the predetermined value.

The invention is explained in more detail below with reference to anexemplary embodiment shown in the drawing. With regard to the details ofa spectacle-lens edging machine which is designed according to theinvention, reference is made to DE 196 16 536 A1 of the same applicant.

Of a spectacle-lens edging machine, a cutting tool in the form of agrinding-wheel stack 1 is shown, a spectacle-lens holding shaftconsisting of two halves 2, 3 being arranged relative to thegrinding-wheel stack 1 in an axially parallel manner. A spectacle lens 5provided with a block or sucker 4 is clamped in place between the halves2, 3 of the spectacle-lens holding shaft. The form grinding of thespectacle lens 5 is effected in such a way that the spectacle lens 5 isset in slow rotation by the spectacle-lens holding shaft 2, 3, while thegrinding-wheel stack 1 rotates at high speed. In the process, thedistance between the spectacle-lens holding shaft 2, 3 with the clampedspectacle lens 5 and the grinding-wheel stack 1 is varied in acontrolled manner by means of a motion drive 15 in accordance with theshape of the spectacle lens to be machined. It is also possible, withthe grinding-wheel stack 1 stationary, to move the spectacle-lensholding shaft 2 relative to the grinding-wheel stack 1.

The spectacle-lens holding shaft 2, 3 is driven by a geared motor 6 viagear sets 7, 8.

During the form machining, the spectacle lens 5 must be rotated by thegeared motor 6 with varying torque, which depends on the cuttingpressure between the grinding-wheel stack 1 and the spectacle lens 5 andon the respective radius of the spectacle lens 5. If this torque exceedsa known empirical value, the spectacle lens 5 clamped in place betweenthe halves 2, 3 of the spectacle-lens holding shaft may slip through,which on no account must occur if the spectacle lens to be machined hasa lower segment oriented at a precise angle with regard to the opticalaxis or a cylindrical or prismatic cut whose axis position relative tothe arrangement of the spectacle lens in the spectacle frame ispredetermined.

The maximum torque which can be transmitted in each case depends on thematerial of the spectacle lens and on its surface treatment. Highlyanti-reflective spectacle lenses, for example, have an especially lowcoefficient of friction relative to holding devices on thespectacle-lens holding shaft or mounted blocks or suckers.

The housing of the geared motor 6 is mounted in a rotatable manner inthe spectacle-lens edging machine and is supported on springs 10 via anextension 9. These springs are arranged between fixed, adjustableabutments 11, so that an adjustable prestress can be imparted to thesprings 10, this prestress being a measure of the torque which can betransmitted by the geared motor 6 before the housing of the geared motor6 is rotated against the force of the springs 10. A position encoder 12is arranged parallel to the springs 10 and interacts with a pin 13. If acertain, adjustable torque of the geared motor 6 is now exceeded, adeflection of the pin 13 relative to the position encoder 12 occurs.This deflection is measured and transmitted to a control device 14,which reduces the rotational speed of the geared motor 6, if need beuntil it is stopped.

For example, the prestress of the springs 10 can be set in such a waythat a deflection of the pin 13 occurs starting from a certain torquewhich is still below the permissible torque, this deflection bringingabout a proportional reduction in the rotational speed of the gearedmotor 6 via the position encoder 12 and the control device 14. If acertain deflection, i.e. a certain, higher torque, is achieved, thecontrol device 14 can stop the geared motor 6, and this geared motor 6remains stopped for as long as the predetermined torque is exceeded.

An especially simple embodiment of the invention may consist in the factthat, instead of a position encoder, a limit switch is arranged in theregion of the extension 9 or pin 13. This limit switch interrupts thepower supply to the geared motor 6 if and as long as a torque which canbe set by the prestress of the springs 10 is exceeded. As soon as thetorque drops to the set value or drops below it, the limit switch closesagain, and the geared motor 6 continues to rotate the spectacle-lensholding shaft 2, 3.

As long as the torque acting on the spectacle-lens holding shaft 2, 3 isless than the predetermined value, continuous grinding of the peripheraledge of the spectacle lens with uninterrupted rotation of thespectacle-lens holding shaft 2, 3 is effected, whereas intermittentgrinding is always effected for as long as this set torque is exceeded.

What is claimed is:
 1. A method of form machining the peripheral edge ofspectacle lenses having a driven spectacle-lens holding shaft holdingthe spectacle lens and a cutting tool movable in a controlled manner inrelation to the spectacle-lens holding shaft, comprising the steps:measuring the torque of the spectacle-lens holding shaft, form machiningthe peripheral edge of the rotating spectacle lens with apredeterminable, adjustable force of the cutting tool if and as long asthe torque is less than or equal to a predetermined, set value, reducingthe cutting-tool force acting on the spectacle lens by reducing therotational speed of the spectacle-lens holding shaft, if need be untilit stops, if and as long as the torque exceeds the predetermined value,wherein the rotational speed of the spectacle-lens holding shaft isreduced in proportion to the amount by which the predetermined value ofthe torque is exceeded.
 2. A method of form machining the peripheraledge of spectacle lenses having a driven spectacle-lens holding shaftholding the spectacle lens and a cutting tool movable in a controlledmanner in relation to the spectacle-lens holding shaft, comprising thesteps: measuring the torque of the spectacle-lens holding shaft, formmachining the peripheral edge of the rotating spectacle lens with apredeterminable, adjustable force of the cutting tool if and as long asthe torque is less than or equal to a predetermined, set value, reducingthe cutting-tool force acting on the spectacle lens by reducing therotational speed of the spectacle-lens holding shaft, if need be untilit stops, if and as long as the torque exceeds the predetermined value,wherein the spectacle-lens holding shaft is stopped for as long as apredetermined value of the torque is exceeded.
 3. An apparatus for formmachining the peripheral edge of spectacle lenses, having a drivenspectacle-lens holding shaft holding the spectacle lens and a cuttingtool movable in a controlled manner in relation to the spectacle-lensholding shaft, a measuring device for measuring the torque of thespectacle-lens holding shaft, and a control device for the force, actingon the spectacle lens, of the spectacle-lens cutting tool by reducingthe rotational speed of the spectacle-lens holding shaft, if need beuntil it stops, if and as long as the torque exceeds a predetermined setvalue, wherein the measuring device for measuring the torque has anelastic support of the rotatably mounted housing of the drive motor forthe spectacle-lens holding shaft.
 4. The apparatus as claimed in claim3, in which a position encoder interacting with the control device isarranged on the elastic support, and the control device brings about areduction in the rotational speed of the spectacle-lens holding shaft oran increase in the distance between the cutting tool and thespectacle-lens holding shaft in proportion to an amount by which ameasured displacement of the position encoder exceeds a predeterminedvalue.
 5. The apparatus as claimed in claim 3, in which the elasticsupport is prestressed in such a way that a movement does not occuruntil the prestress is exceeded, and in which an electrical contactinteracts with the elastic support, this electrical contact interruptingthe power supply to the drive motor when the prestress is exceeded.